Among many other contributions to oral health, saliva helps to maintain an ecological balance within the diverse oral biofilm microbiota through fostering colonization by harmless commensal bacteria. Because the oral cavity serves as a port of entry for pathogenic microorganisms to both gastrointestinal and respiratory tracts, legitimate interest exists to dissect salivary components that interact with commensal bacteria from those that are bound by putative extraoral pathogens. Preliminary data suggest that there exists additional not yet well understood complexity in the structural presentation or substitution of sialic acids on salivary glycoproteins that are likely to influence adhesin-mediated bacterial binding by both, commensal oral streptococci and extraoral pathogens. Considering that more than 40 different structural subtypes of sialic acids exist in nature and are known to mediate a wide variety of physiological and pathological processes, including recognition by viruses and other pathogens, thus far little work has been done to determine which structural sialic acid subtypes are present on glycoproteins in human saliva, and which are specifically recognized by oral commensal streptococci in comparison to sialic acid binding pathogens such as H. pylori or S. pneumoniae. We hypothesize that sialic acid substitution by O-acetyl groups influences streptococcal binding. The results are expected to shed additional light on the surprisingly narrow tissue and host tropism of certain viridans streptococci, of which some thrive exclusively within the human oral cavity. Identification of the structural requirements for bacterial binding to sialic acids will be relevant not only to saliva and susceptibility to oral diseases, such as caries and periodontitis, but may also be readily applied to the interaction of pathogenic microorganisms with other bodily secretions and mucous-covered epithelia in respiratory, digestive and genital tracts. Moreover, once the exact sialic acid recognition motifs for certain microbial adhesins are determined, it will become possible to design carbohydrate analogues as drugs to prevent colonization by undesired bacteria. The specific aims of the project are to: 1. Define how 9-O-acetylated sialic acids are distributed among members of the 2-D salivary glycoproteome. 2. Determine the structural subtypes of sialic acids recognized by the corresponding adhesins and demonstrate that 9-O-acetylation of sialic acids determines streptococcal binding to salivary glycoproteins. These aims will be achieved by a combination of glycobiological and proteomics methods. PUBLIC HEALTH RELEVANCE: The proposed study sets out to add significance to the oral cavity being a main way of entry to both gastrointestinal and respiratory tracts by investigating the discriminating role of saliva in fostering oral colonization by a beneficial commensal microflora versus preventing infection by pathogenic microorganisms. This research is relevant to human health because the findings are expected to ultimately help understand tissue tropism of bacteria to the mouth and other mucus-lined tissues. Deciphering the molecular mechanisms underlying this tropism is expected to pave the way for future pharmacological prevention of colonization by undesired bacteria.